180' rule

The 180 degree rule of shooting and editing is when the camera is kept on one side of the action.

 

 This image illustrates the rule, in order for the audience to easily follow the conversation/action happening in the sequence the camera should stay on one side of the 180 degree line. One technique used in conversation scenes is a 'shot reverse shot' each time they are used the eyelines of the characters will match if the 180 degree rule is broken the audience could become confused and be distracted from the dialogue due to trying to figure out who is looking at who. The only time the 180 degree rule can be broken is when it is a continuous shot around a character the camera must be recording well before and after it has crossed the line.